The present invention relates to conveyor systems for stacking and transporting sheet material, and more particularly to lift sections of the conveyor system.
Conveyors play a critical role in the packaging and transport of sheet material, such as corrugated sheet material. Conveyors carry the newly manufactured sheet to various stations where stacks of sheets are formed and eventually loaded onto pallets for shipment. In order to optimize control over the sheets and stacks of sheets, many conveyors incorporate powered drive rollers to propel and direct the product.
In some conveyor systems, loads are formed on a conveyor section that can be raised and lowered. This conveyor section, commonly called a lift table, is an ergonomic solution for manual load forming. The lift table is initially elevated to a comfortable work height allowing an operator to manually transfer a bundle of sheet material onto the conveyor surface without bending. As each tier is positioned on the lift table, the table is lowered so that the new work surface is again at a comfortable working height. As new tiers of sheet material are added the lift table is progressively lowered until the maximum load is reached. The lift table is then raised so that the entire newly formed load is aligned with a discharge conveyor.
Thus, as shown in FIGS. 1-2, a conveyor system 10 includes a unit load station 15 and a discharge conveyor 11 adjacent each other. The unit load station includes a conveyor lift table assembly 20 flanked by retractable backstops B adjacent two sides of the lift table assembly. The backstops may be extended into a position contiguous with the sides of the lift table to facilitate load building. Once the load is fully built it can be discharged away from the lift table assembly 20 on the conveyor 11.
The lift table assembly 20 includes a conveyor 22, which may be a roller-type conveyor with rollers 24 arranged to support the sheet material load as it is built. Once the load is complete the rollers facilitate transferring the load to the discharge conveyor 11. The conveyor 22 includes a frame 26 supporting the rollers 24 and providing an interface to the lift mechanism 30. The lift mechanism is typically a scissors-type mechanism with two arms 32 and 36. The arm 32 is pivotably supported at a mount 33 on a base 46 supported on a floor. The opposite end of the arm carries a roller 34 that is adapted to roll on the underside of the conveyor frame 26. The other arm 36 is pivotably supported at one end on a mount 37 on the underside of the conveyor frame 26. The opposite end of the arm 36 carries a roller 38 that rolls on the base 46. The two arms are pivotably attached at a fulcrum point 39 to permit the scissors motion of the two arms. The arms are activated by a drive member 40, which may be in the form of a pneumatic or hydraulic cylinder. The drive member 40 is pivotably supported at a mount 44 on the base 46, while the push rod 42 of the drive member is pivotably connected to one of the arms 36 between its pivot mount 37 and the fulcrum 39. Extension of the push rod 42 causes the lift mechanism 30 to retract while retraction of the push rod causes the mechanism to extend or rise.
Although the scissors lift mechanism is an inexpensive and simple means to raise and lower the conveyor lift table 20, it carries with it the risk of objects or personnel entering the cavity beneath the conveyor 22 during operation of the lift mechanism. Typically a vinyl skirt extends from the conveyor 22 to provide a visual warning to stay clear of the area under the conveyor. The skirt may also prevent entry of light-weight objects. However, the skirt does not prevent an operator from moving his/her leg or foot under the conveyor, and does not prevent more massive articles from sliding or rolling underneath during operation. Consequently, there is a need for a system to protect the area underneath the conveyor 22 during operation of the lift table.